This invention relates in general to a method for decreasing heat transfer from production of a well to the geological formation into which the well bore extends.
An oil or gas well normally has one or more strings of casing extending into a well that are cemented in place. The production casing is perforated in an earth formation bearing hydrocarbons. A string of production tubing extends into the production casing. Often, a packer will seal the lower end of the tubing to the production casing at a point above the perforations. Oil and/or gas is produced through the production tubing to the surface.
In arctic regions, a cold permafrost formation layer often extends to depths of 2,000 feet below the surface. Liquids and gases passing through this cold layer may be cooled to the point that viscosity increases and hydrates and condensates begin to form. Water freezing can result, restricting well production.
In temperate zone gas wells, gas expansion through downhole chokes can result in lowering gas temperatures to the level that some of the same problems encountered in arctic wells began to appear. In low pressure, wet gas wells, condensation can form suspended slugs of condensate within the production tubing or casing annulus. This condensate significantly reduces the well""s production.
It is known that heating the liquid or gas flowing through the production tubing can retard the undesirable effects mentioned above. One heating device uses resistance type electrical cable suspended within the production tubing or strapped to the outside diameter of the production tubing. While such will retard the cooling of the liquid, much of the heat will be lost through the tubing annulus to the geological formation. This lost heat is not available to increase the temperature of the produced liquid or gas and significantly increases heating costs. It is also known to thermally insulate at least portions of the production tubing in various manners to retard heat loss, however improvements are desired.
In this invention, temperature loss of fluid being produced in a well is reduced by providing a fluid of low thermal conductivity in the tubing annulus. The tubing annulus extends radially between the casing and the production tubing and axially from a packer just above the perforations to the wellhead. In one method, the low thermal conductivity fluid is provided by drawing at least a partial vacuum on the tubing annulus. This reduces the amount of air left in the tubing annulus, thereby lowering the thermal conductivity. Preferably about 27xe2x80x3 to 29xe2x80x3 of vacuum is drawn on the tubing annulus.
In another aspect of the invention, providing low thermal conductivity fluid in the tubing annulus is accomplished by substantially filling the tubing annulus with a hydrocarbon liquid. The hydrocarbon liquid should be viscous, preferably at least 1,000 centipoise at 100xc2x0 F. Also, preferably the tubing is centered in the well with a plurality of centralizers that extend between the casing and the tubing.